1. Statement of the Technical Field
The present invention relates to the field of antennas, and more particularly to dynamically adjustable reflectors and sub-reflectors using fluidic dielectrics.
2. Description of the Related Art
Typical satellite antenna systems use either parabolic reflectors or shaped reflectors to provide a specific beam coverage, or use a flat reflector system with an array of reflective printed patches or dipoles on the flat surface. These “reflectarray” reflectors used in antennas are designed such that the reflective patches or dipoles shape the beam much like a shaped reflector or parabolic reflector would, but are much easier to manufacture and package on a spacecraft. These antennas will be initially configured to reduce side lobes or to avoid reflecting side lobes.
Since satellites typically are designed to provide a fixed satellite beam coverage for a given signal and may be limited in bandwidth by the structure of the reflectors. For example, Continental United States (CONUS) beams are designed to provide communications services to the entire continental United States. Once the satellite transmission system is designed and launched, changing the beam patterns to improve the operational bandwidth would be difficult. Additionally, antennas using feeds operating over a range of frequencies may also experience performance degradation due to appreciable side lobes in given frequency range. The side lobes are usually a result of edge diffraction of the radiation from the feed. The diffraction spreads the radiation into unwanted directions and causes interference with other electronic systems. A proper edge treatment can reduce the effect of the side lobes and improve overall antenna performance. Commonly used methods include serrated edges and rolled back edges. Another system by Ohio State University uses sputtered carbon on the surface of the reflector to provide different values of resistance. All these solutions are fine for fixed configurations that don't require adjustments. Even fixed configurations may require adjustments over time for various reasons such as environmental conditions or normal wear and tear causing system degradation.
A microwave antenna projects a traveling microwave onto an aperture in free space. The electromagnetic field at each point as defined by the projection can be considered a source of a secondary spherical wave known as Huygens' wavelet. The envelope of all Huygens' wavelets emanating from the antenna aperture at any instant of time is then used to describe the transmitting electromagnetic radiation from the antenna at a later instant of time. This is known as the famed Huygens-Fresnel Principle and mathematically can be represented by the Rayleigh-Sommerfeld diffraction formula, which is a Fourier type integration. The assumption with fixed antennas is that their aperture must be finite in size which imposes a window on the Rayleigh-Sommerfeld diffraction formula for an untreated microwave antenna. It is well known in Fourier analysis that a window discontinuous at the aperture edges leads to high side lobes. These side lobes can be reduced by employing smooth tapered windows before evaluating the Fourier transformation. The edge treatment of microwave antennas corresponds to imposing a smooth tapered window onto the Rayleigh-Sommerfeld diffraction formula. The serrated and rolled edge treatments differ in methods of tapering. The former is restricted to the magnitude tapering of the electromagnetic field at the aperture of a microwave antenna, and the latter is mainly confined to phase tapering with little controls on the magnitude. The electromagnetic field has two independent components—magnitude and phase. Any abrupt change in either component will lead to high side lobes. Both serrated and rolled edge treatments are restricted to a single component, neglecting the other. The abrupt change can not be optimally removed with either of these two methods. The present invention can treat both components simultaneously, hence provide a better optimum method than either of them, therefore leading to much better side lobe reduction.
Passive reflectors are generally broadband structures, and in fact the principal beam direction from a reflector system is typically independent of frequency. However, beamwidth and sidelobe directions are not independent of frequency. In mathematical terms, this is because the domain of the Rayleigh-Sommerfeld integration scales with wavelength. Thus a shaped beam designed to cover the CONUS will be correctly sized at only a single frequency, and will be too large at lower frequencies, and too small at higher frequencies. In addition, although the reflector functions over a broad frequency range, the radiation pattern of the feed structure is typically frequency dependent, and the optimum reflector size and shape for a particular feed changes with frequency. Reflectarrays have the additional complication that the array elements will have frequency dependence. The combination of all these factors limits the frequency range of conventional shaped beam reflector designs.
The need to change the beam pattern provided by the satellite and further account for side lobe effects has become more desirable with the advent of direct broadcast satellites that provide communications services to specific areas and possibly on different frequencies ranges. Without the ability to change beam patterns and coverage areas as well as to flexibly use multiple frequency ranges, additional satellites must be launched to provide the services to possible future subscribers, which increases the cost of delivering the services to existing customers.
Some existing systems are designed with minimal flexibility in the delivery of communications services. For example, a symmetrical Cassegrain antenna that uses a movable feed horn, defocuses the feed and zooms circular beams over a limited beam aspect ratio of 1:2.5. This scheme has high sidelobe gain and low beam-efficiency due to blockage by the feed horn and the subreflector of the Cassegrain system. Further, this type of system splits or bifurcates the main beam for beam aspect ratios greater than 2.5, resulting in low beam efficiency values. Other systems attempt to alter beam width and gain by using multiple feed horns. In any event, most of these systems will have a main reflected signal that will be interfered with by a side lobe of the radiator or feed horn.
In another system as shown in FIG. 1, a dynamic reflector surface comprising an array of tunable reflective surfaces is used instead of a fixed reflector surface. Each element of the array can be tuned separately to change the phase during the process of reflection, and thus the beam pattern generated by the array of tunable reflectors can be changed in-flight in a simple manner. Each reflecting element in the array is a horn reflecting device which reflects an electric field emanating from a single feed horn. Each horn in the array has the capability of changing the phase during the process of incidence and reflection. This phase shift can then be used to change the shape of the beam emanating from the array. The phase shift can be incorporated by either using a movable short or by using a variable phase-shifter inside the horn and a short. By using “phase-shifting” which can be controlled on-orbit, a relatively simple reconfigurable antenna can be designed. This approach is much simpler than an active array in terms of cost and complexity.
More specifically, FIG. 1 illustrates a front, side, and isometric view of the existing horn reflect array as described in U.S. Pat. No. 6,429,823. Reflect array 200 is illuminated with RF energy from feed horn 202. Reflect array 200 comprises a plurality of reflective elements 204 that are configured in a reflector array 206. Side view 208 shows that feed horn 202 is pointed at the open end 210 of reflective element 204. Side view 208 also shows that reflector array 206 can be a curved array. Further, front view 212 and isometric view 214 show that reflective elements 204 can be placed in a circular arrangement for reflector array 206. Each reflective element 204 reflects a portion of the incident RF energy, and by changing the respective phase for each reflective element 204, the respective phase of the portion of the reflected RF energy for each respective reflective element 204 can be changed. By changing the phase of each portion of the reflected RF energy, different beam patterns can be generated by the horn reflect array. Although the reflector array 206 provides lower non-recurring costs for a satellite and can generate a plurality of different shaped beam patterns without reconfiguring the physical hardware, e.g., without moving the location of the feed horn 202 and the reflective elements 204 in the reflector array 206, the design is still more complicated than needed to obtain similar results. Fortunately, the only thing that must change from mission to mission using the reflect array 200 is the programming of the reflective elements 204.
In any event, a programmable array such as the reflector array 206 can be reconfigured on-orbit. Satellites using the reflector array 206 can be designed for use in clear sky conditions, and, when necessary, the beams emanating from the reflector array 206 can be shaped to provide higher gains over geographic regions having rain or other poor transmission conditions, thus providing higher margins during clear sky conditions.
It can be seen, then, that there is a need in the art for an antenna system that can be alternatively reconfigured in-flight to reduce the effects of side lobes from one or more sources (feeds) without the need for complex systems as discussed above. It can also be seen that there is a need in the art for a communications system that can be reconfigured in-flight that has high beam-efficiencies and high beam aspect ratios. An alternative arrangement for achieving the advantages of the antenna of FIG. 1 and other advantages as will be further described below utilizes fluidic dielectrics in accordance with the present invention.
Two important characteristics of dielectric materials are permittivity (sometimes called the relative permittivity or εr) and permeability (sometimes referred to as relative permeability or μr). The relative permittivity and permeability determine the propagation velocity of a signal, which is approximately inversely proportional to √{square root over (με)}. The propagation velocity directly affects the electrical length of a transmission line and therefore the amount of delay introduced to signals that traverse the line.
Further, ignoring loss, the characteristic impedance of a transmission line, such as stripline or microstrip, is equal to √{square root over (Ll/Cl)} where Ll is the inductance per unit length and Cl is the capacitance per unit length. The values of Ll and Cl are generally determined by the permittivity and the permeability of the dielectric material(s) used to separate the transmission line structures as well as the physical geometry and spacing of the line structures.
For a given geometry, an increase in dielectric permittivity or permeability necessary for providing increased time delay will generally cause the characteristic impedance of the line to change. However, this is not a problem where only a fixed delay is needed, since the geometry of the transmission line can be readily designed and fabricated to achieve the proper characteristic impedance. Analogously, wave propagation delays and energy beam patterns through dielectric materials in reflector and/or sub-reflector based antenna systems are typically designed accordingly with a fixed dielectric permittivity or permeability. When various time delays are needed for specific energy shaping or beam forming requirements, however, such techniques have traditionally been viewed as impractical because of the obvious difficulties in dynamically varying the permittivity and/or permeability of a dielectric board substrate material. Accordingly, the only practical solution has been to design variable delay lines using conventional fixed length RF transmission lines with delay variability achieved using a series of electronically controlled switches. Such schemes would be impracticable and overly complicated for a reflector or sub-reflector based antenna.